The normal development of the complex circuitry in the nervous systems requires that synapses not only be formed precisely, but that synapses be appropriately remodeled and refined during early development. We are rapidly moving toward a complete catalogue of synaptic proteins. In addition, many of the protein-protein interactions that network synaptic molecules into pre- and postsynaptic complexes have been defined. A major challenge, now, is to elucidate the signaling mechanisms that initiate, maintain, and then disassemble these extraordinary protein complexes that make up the synapse. These developmental signaling systems will be fundamental to our understanding of how the wiring of the nervous system is established, and they may be relevant to the activity-dependent modulation of neural circuitry. This proposal contributes toward this goal by defining a synaptic signaling system that appears to control postsynaptic differentiation including the clustering of glutamate receptors and the specialization of the postsynaptic membrane. Here we propose to characterize a synaptic signaling system that includes both the SH3 adaptor protein Dock and the serine threonine kinase dPak. During axon guidance in Drosophila, Dock and dPak bind and signal to the actin cytoskeleton. At the synapse, however, our data place Dock and dPak into distinct signaling pathways that converge to control the development, and possibly the maintenance, of the postsynaptic apparatus. Here we propose a characterization of Dock and dPak signaling at the synapse using genetic, molecular, anatomical and electrophysiological tools at the Drosophila neuromuscular junction. We will then move beyond these two molecules based on previous biochemical data to explore the role of additional signaling molecules that participate in Dock and dPak signaling. The ultimately goal will be to define the entire signaling system that controls the development and maintenance of a glutamatergic synapse from the signaling event at the plasma membrane to the downstream control of receptor localization and synaptic cytoskeletal rearrangement.